Set of piston compressors and method for oil compensation

ABSTRACT

A set of piston compressors ( 2, 4, 6, 8 ) for use in a refrigeration system comprises a fluid connection ( 10 ) between the crank cases of the piston compressors ( 2, 4, 6, 8 ) for allowing oil compensation between said piston compressors ( 2, 4, 6, 8 ); and a control unit ( 12 ) which in operation monitors the running time of each piston compressor ( 2, 4, 6, 8 ) and, in case all piston compressors ( 2, 4, 6, 8 ) are running, subsequently interrupts the operation of a respective piston compressor ( 2, 4, 6, 8 ) for a predetermined period of time for oil compensation if its running time reaches a predetermined maximum running time.

This application is entitled to the benefit of, and incorporates byreference essential subject matter disclosed in PCT Application No.PCT/EP2007/009044 filed on Oct. 18, 2007.

BACKGROUND OF THE INVENTION

1. Technical Field

The invention relates to a set of piston compressors for use in arefrigeration system and to a method for oil compensation in a set ofpiston compressors.

2. Background Information

Refrigerating circuits often comprise a set of compressors which may belocated in a separate machine room. The individual compressors areselectively switched on and off in order to match the actual need forcooling capacity. However, a compressor running continuously for a longperiod of time may run out of oil needed for lubrication, resulting indamage of the compressor. Thus, conventionally sets of compressors areswitched off completely in certain intervals to allow for oilcompensation. This results in a discontinuity of the output of the setof compressors over time compromising the performance of therefrigerating circuit.

Accordingly it would be beneficial to provide an improved set of pistoncompressors which ensures an adequate lubrication and at the same time aconstant output.

SUMMARY OF THE INVENTION

Exemplary embodiments of the invention include a set of pistoncompressors for use in a refrigeration system comprising a fluidconnection between the crank cases of the piston compressors forallowing oil compensation between said piston compressors and a controlunit which in operation monitors the running time of each pistoncompressor, and, in case all piston compressors are running,subsequently interrupts the operation of a respective piston compressorfor a predetermined period of time for oil compensation if its runningtime reaches a predetermined maximum running time.

Exemplary embodiments of the invention further include a set of pistoncompressors comprising a fluid connection between the crank cases of thepiston compressors for allowing oil compensation between said pistoncompressors, and a control unit which, in case all piston compressorsare running, monitors the common running time of the piston compressors,and, when the common running time reaches a predetermined maximum commonrunning time, sequentially interrupts the operation of a each pistoncompressor for a predetermined period of time for oil compensation whilethe other compressors continue to run.

Exemplary embodiments of the invention further include a method for oilcompensation in a set of piston compressors comprising the steps ofmonitoring the running time of each piston compressor and, in case allpiston compressors are running, interrupting the operation of a pistoncompressor for a predetermined period of time for oil compensation whenits running time reaches a predetermined time of operation.

Embodiments of the invention further comprise a method for oilcompensation in a set of piston compressors comprising the steps ofmonitoring the running time of the piston compressors and, in case allpiston compressors are running and the running time reaches apredetermined time of operation, interrupting the operation of a firstpiston compressor for a predetermined period of time for oilcompensation; and sequentially interrupting the operation of a each ofthe additional piston compressors for a predetermined period of time foroil compensation.

Embodiments of the invention will be described in greater detail belowwith reference to the enclosed Figures, wherein

DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a set of piston compressors according to a first embodimentof the invention;

FIG. 2 shows an exemplary diagram depicting the operation of the set ofpiston compressors shown in FIG. 1 over time;

FIG. 3 shows a set of piston compressors according to a secondembodiment of the invention;

FIG. 4 shows an exemplary diagram depicting the operation of the set ofpiston compressors shown in FIG. 3 over time;

FIG. 5 shows a set of piston compressors according to a third embodimentof the invention; and

FIG. 6 shows an exemplary diagram depicting the operation of the set ofpiston compressors shown in FIG. 5 over time.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a set of four piston compressors 2, 4, 6, 8 according to afirst embodiment of the invention. The crank cases of said pistoncompressors 2, 4, 6, 8 are respectively filled up to a certain level 32,34, 36, 38 with a lubricating oil 28. Each of the piston compressors 2,4, 6, 8 comprises an inspection glass 14 for inspecting the level 32,34, 36, 38 of the lubricating oil 28 within the respective crank case. Afirst end of a respective oil conduit 40, 42, 44, 46 opens into each ofthe crank cases at approximately the same height at which the inspectionglass 14 is positioned. A second end of each of said oil conduits 40,42, 44, 46 opens into a fluid connection 10 fluidly connecting the crankcases of the piston compressors 2, 4, 6, 8 with each other for allowingoil compensation. The oil conduits 40, 42, 44, 46 can open into thefluid connection 10 from the bottom, from the top or from the side.Thus, when all piston compressors 2, 4, 6, 8 are stopped, the levels 32,34, 36, 38 of the oil 28 within the crank cases will adjust toapproximately the same height within the visible range of each of theinspection glasses 14. Traps/siphon traps 26 are arranged in oilconduits 40, 42, 44, 46 for providing a gas-tight seal in order toprevent gas from flowing from the compressors 2, 4, 6, 8 into the fluidconnection 10. The traps/siphons may be arranged close to thecompressors 2, 4, 6, 8, close to the manifold 10 or in the manifold 10itself. A control unit 12 is connected to each of the compressors 2, 4,6, 8 for controlling their operation.

The number of four compressors 2, 4, 6, 8 as shown in FIG. 1 is onlyexemplary and any number of compressors 2, 4, 6, 8 which is consideredsuitable for providing the desired output may be used in a set ofcompressors 2, 4, 6, 8 according to embodiments of the invention.

FIG. 2 shows an exemplary diagram depicting the operation of the set ofpiston compressors 2, 4, 6, 8 over time. The time t is plotted on thex-axis, while the respective modes of operation of the four compressors2, 4, 6, 8 are plotted on the y-axis.

At time t0 all compressors 2, 4, 6, 8 stand still. At time t1 the firstcompressor 2 is started due to a need of compression/refrigeration. Attime t2 the second compressor 4 is started as well since additionalcompression/refrigeration is needed. At time t3 the first compressor 2is running for a predetermined maximum running time T0, thus it needs tobe switched off for oil compensation. As not all compressors 2, 4, 6, 8are running at this moment in time t3, a basic load switching operationis executed, i.e. the first compressor 2 is switched off and the thirdcompressor 6, which has not been running before, is switched on takingover the workload/performance of the first compressor 2. The second andfourth compressors 4, 8 are not affected by this basic load switchingoperation and maintain their respective status of operation.

At time t4 the required output is reduced and the operation of thesecond compressor 4 is no longer needed, thus the second compressor 4 isswitched off before it reaches the predetermined maximum running timeT0. At time t5 the third compressor 6 is switched off as well and allcompressors 2, 4, 6, 8 are stopped again.

The mode of operation during the interval from t0 to t5 is an examplefor oil compensation by basic load switching which is used when not allcompressors 2, 4, 6, 8 are running.

At time t6 the first compressor 2 is started again to a need ofcompression/refrigeration. At times t7, t8 and t9 the second, third andfourth compressors 4, 6, 8 are started, respectively, to an stepwiseraised need of compression/refrigeration so that at time t9 all fourcompressors 2, 4, 6, 8 are running. At time t10 the first compressor 2is stopped for a short period of time ΔT. However, since said shortperiod of time ΔT is shorter than the predetermined period of time foroil compensation TC the counting of the operation time of the firstcompressor 2 is not set back to zero but continues to count from thepoint in time t6, when the first compressor 2 has been started after ithas not been standing still for a longer period of time than thepredetermined time for oil compensation TC. Thus, the first compressor 2reaches the predetermined maximum running time T0 at time t11 and it isswitched off for oil compensation. After the predetermined time for oilcompensation TC has expired the first compressor 2 is started again.

At times t12, t13 and t14 the second, third and fourth compressors 4, 6,8 reach their predetermined maximum running time TO and are respectivelyswitched off for a predetermined time for oil compensation TC. It is tobe noted that the determination if a compressor 2, 4, 6, 8 reaches thepredetermined maximum running time TO is done independently for eachrespective compressor 2, 4, 6, 8, and thus each of the compressors 2, 4,6, 8 is switched off basically independently of the other compressors 2,4, 6, 8. However, if a compressor 2, 4, 6, 8 reaches the predeterminedmaximum running time TO when another compressor 2, 4, 6, 8 has alreadybeen stopped for oil compensation or during a delay time TS after saidother compressor 2, 4, 6, 7 has been started again, said compressor 2,4, 6, 8 is not stopped immediately but only after the predetermineddelay time TS after the other compressor 2, 4, 6, 8, which has beenstopped before, has been started again. In other words, two compressors2, 4, 6, 8 are not out of operation at the same time for oilcompensation. This ensures that the set off piston compressors 2, 4, 6,8 can provide sufficient output at any time.

The operation within the interval from t6 to t14 is an example for oilcompensation according to an embodiment of the invention when allcompressors 2, 4, 6, 8 are running.

FIG. 3 shows a set of four piston compressors 18, 20, 6, 8 according toa second embodiment of the invention. Like reference signs correspond tolike features which will not be discussed in detail again. In theembodiment shown in FIG. 3 two of the compressors 18, 20, 6, 8, namelythe compressors 18, 20 are designed as speed controllable compressors18, 20 which are selectively connectable to a speed control 22 viaswitch 24. The speed control 22 and the switch 24 are controlled by thecontrol unit 12. By controlling the speed of at least one of the speedcontrollable compressors 18, 20 the output of the set piston compressors18, 20, 6, 8 can be adjusted finely to match exactly the desired output.I.e., the output is adjusted coarsely in a first step by controlling thenumber of the compressors 18, 20, 6, 8 running and finely in a secondstep by controlling the speed of at least one of the speed controllablecompressors 18, 20.

FIG. 4 shows an exemplary diagram of operation of the set of pistoncompressors 18, 20, 6, 8 over time. The time t is plotted against thex-axis, while the respective modes of operation of the four compressors18, 20, 6, 8 and of the switch 24 are plotted against the y-axis.

At time t0 all compressors 18, 20, 6, 8 stand still and the switch 24connects the second speed controllable compressor 20 to the speedcontrol 22. At time t1 said second speed controllable compressor 20 isstarted. At time t2 the third compressor 6 running at constant speed isstarted, too. The speed of the speed controllable compressor 20 isalways adjusted so that the set of piston compressors 18, 20, 6, 8generates exactly the desired output.

At time t3 the second speed controllable compressor 20 reaches thepredetermined maximum running time TO. Thus a basic load switchingoperation is executed as follows:

The speed of the second speed controllable compressor 20 is reduced tozero, some, preferably three, seconds later the second speedcontrollable compressor 20 is shut off. Some, preferably three, secondslater the switch 24 connects the first speed controllable compressor 18to the speed control 22, and some, preferably three, seconds later, saidfirst speed controllable compressor 18 is started and the speed of saidfirst speed controllable compressor 18 is gradually increased, so thatsaid first speed controllable compressor 18 takes over the workload andperformance of the second speed controllable compressor 20 which hasbeen stopped for oil compensation.

At time t4 the third compressor 6 running at constant speed reaches thepredetermined maximum running time TO. Thus it is stopped and the fourthcompressor 8 running at constant speed is started taking over theworkload of the stopped third compressor 6, as already described withreference to FIG. 2.

At time t5 the third compressor 6 running at constant speed is switchedon in addition to the fourth compressor 8 running at constant speed inorder to increase the total output of the set of compressors 18, 20, 6,8.

At time t6 the second speed controllable compressor 20 is also switchedon in order to increase the output even further. It is to be noted thatthe speed control 22 remains connected to the first speed controllablecompressor 18 at time t6 so that the second speed controllablecompressor 20 is running with constant speed.

At time t7 the first speed controllable compressor 18 reaches thepredetermined maximum running time TO. Thus a basic load switchingoperation is executed as follows:

First, the speed of the second speed controllable compressor 20 runningat constant speed, is reduced, for example to 35 Hz and then saidcompressor 20 is shut off.

Then the speed of the first speed controllable compressor 18 is reducedto zero, some, preferably three, seconds later the second speedcontrollable compressor 18 is shut off. Some, preferably three, secondslater the switch 24 connects the second speed controllable compressor 10to the speed control 22, and some, preferably three, seconds later, saidsecond speed controllable compressor 20 is started and the speed of saidsecond speed controllable compressor 20 is gradually increased until itreaches the original speed of the first speed controllable compressor 18before the basic load switching operation has been started, so that saidsecond speed controllable compressor 20 takes over the workload andperformance of the first speed controllable compressor 18 which has beenstopped for oil compensation.

After the predetermined time for oil compensation TC has expired, thefirst speed controllable compressor 18 is started again. A predetermineddelay time TS after the first speed controllable compressor 18 has beenstarted again, the second speed controllable compressor 20 is stoppedfor a predetermined time TC for oil compensation at time t8 although ithas not reached the predetermined maximum running time TO, yet. However,in the mode of operation shown in FIG. 4, the second speed controllablecompressor 20 is always stopped for oil compensations after apredetermined delay time TS after the first speed controllablecompressor 18 has been stopped for oil compensation and has been startedagain.

At times t9 and t10 the fourth and third compressors 8, 6 running atconstant speed respectively reach their predetermined maximum runningtime TO and are therefore respectively stopped for a predetermined timeTC for oil compensation as already described with reference to FIG. 2.

At time t11 the first speed controllable compressor 18 reaches thepredetermined maximum running time TO. Thus it is stopped for oilcompensation and the switch 24 connects the first speed controllablecompressor 18 to the speed control 22. The first speed controllablecompressor 18 is started again after the predetermined time TC for oilcompensation has expired.

After a predetermined delay time TS after the first speed controllablecompressor 18 has been started again, the second speed controllablecompressor 20 is stopped at time t12 for a predetermined time TC for oilcompensation.

At times t13 and t14, the third and fourth compressors 8, 6 running atconstant speed respectively reach the predetermined maximum running timeTO and are respectively stopped for a predetermined time TC for oilcompensation as already described with reference to the firstembodiment.

FIG. 5 shows an alternative embodiment of the set of piston compressors2, 4, 6, 8 shown in FIG. 1. Like features are designated by the samereference signs and will not be discussed in detail again.

In the embodiment shown in FIG. 5 the fluid connection 10 connecting thecrank cases of the compressors 2, 4, 6, 8 is formed as a circularmanifold. The fluid connection 10 is provided with four valves 16fluidly separating the compressors 2, 4, 6, 8 from each other. Thevalves 16 are designed as one-way, non-return valves 16 restricting theflow within the fluid connection 10 to a predetermined direction A. Inthis embodiment the order of stopping the compressors 2, 4, 6, 8 for oilcompensation corresponds to the direction of oil circulation A as it isdefined by the one-way valves 16.

FIG. 6 shows an exemplary diagram of the operation of the set of pistoncompressors 2, 4, 6, 8 over time. Again, the time t is plotted againstthe x-axis, while the respective modes of operation for the fourcompressors 2, 4, 6, 8 are plotted against the y-axis.

At time t0 all compressors 2, 4, 6, 8 stand still. At time t1 the firstcompressor 2 is started. At times t2, t3 and t4 the second, third andfourth compressor 4, 6, 8 are respectively started as well so that allcompressors are running at time t4.

At time t5 the first compressor 2 reaches the predetermined maximumrunning time TO and therefore it is stopped for oil compensation. Afterthe predetermined time for oil compensation TC has expired the firstcompressor 2 is started again.

After a predetermined delay time TS after the first compressor 2 hasbeen started again, the second 4 is stopped for a predetermined time foroil compensation TC at time t6 although it has not yet reached thepredetermined maximum running time. However, in the mode of operationshown in FIG. 6, all compressors 2, 4, 6, 8 will be stopped sequentiallywhen only one of the compressors 2, 4, 6, 8 has reached thepredetermined maximum running time TO. Thus, the third compressor 6 isstopped at time t7, after a predetermined delay time TS after the secondcompressor 4 has been started again and the fourth compressor 8 isstopped at time t8 after a predetermined delay time TS after the thirdcompressor 6 has been started again.

In the mode of operation shown in FIG. 6 the compressors 2, 4, 6, 8 arestopped sequentially in a predetermined sequence in order to allow foroil compensation through the circular manifold in the direction Adefined by the one-way valves 16. It is to be noted that two of thecompressors 2, 4, 6, 8 will not be stopped at the same time but that afurther compressor 2, 4, 6, 8 will be stopped only after a predetermineddelay time TS after the compressor 2, 4, 6, 8 which has been stoppedbefore has been started again.

In an alternative embodiment the order of stopping the compressors 2, 4,6, 8 is reversed every time the predetermined time of the operation ofall compressors 2, 4, 6, 8 is reached. However in such an embodiment thefluid connection 10 may not be provided with one-way valves 16 as thedirection of the oil flow will be reversed every time the predeterminedtime of the operation of all compressors 2, 4, 6, 8 is reached.

In a set of piston compressors according to the exemplary embodiments ofthe invention as described above each piston compressor is stopped for apredetermined period of time for oil compensation when its running timereaches a predetermined maximum running time. Not more than one pistoncompressor is stopped at the same time for oil compensation. Thus, anadequate lubrication of the compressors is ensured while at the sametime a sufficient output is provided continuously over time.

In an embodiment of the invention the set of piston compressorscomprises at least one piston compressor running at constant speed andat least one speed controllable piston compressor. A speed controllablepiston compressor allows to finely adjust the total output of the set ofpiston compressors.

In an embodiment of the invention the set of piston compressorscomprises at least two speed controllable piston compressors which areselectively connectable to a speed control for controlling the speed ofthe compressors. In this embodiment the control unit in operationsequentially interrupts the operation of each of the speed controllablepiston compressors not connected to the speed control for apredetermined period of time for oil compensation after a predetermineddelay time after the first speed controllable piston compressor, whichhas been stopped for oil compensation, has been started again. Thisembodiment allows to finely adjust the output of the set of compressorseven when one of the speed controllable piston compressors is stoppedfor oil compensation and it ensures a sufficient lubrication of all thespeed controllable piston compressors.

In an embodiment of the invention the speed controllable pistoncompressors are stopped sequentially one after the other, and each speedcontrollable piston compressor is stopped for oil compensation after apredetermined delay time after the speed controllable piston compressorwhich has been interrupted before has been started again. This ensuresan appropriate oil compensation between the speed controllable pistoncompressors.

In an embodiment of the invention the control unit in operation startsover in counting the running time of a respective piston compressor,when said piston compressor is stopped for a period of time that islonger than the predetermined period of time for oil compensation. Thisavoids that a compressor is stopped without need for oil compensation.

In an embodiment of the invention the control unit in operation stops afirst piston compressor, which reaches its maximum time of operationwhile a second piston compressor has been stopped for oil compensation,after a predetermined delay time after said second compressor has beenstarted again. This avoids that two piston compressors are stopped atthe same time for oil compensation which would result in a considerableand undesirable reduction of the output of the set of pistoncompressors.

In an embodiment of the invention each piston compressor is stopped foroil compensation after a predetermined delay time after the pistoncompressor which has been stopped before has been started again. Thisensures that a sufficient output is provided at any time, even when oneof the piston compressors is stopped for oil compensation.

In an embodiment of the invention the fluid connection is a circularmanifold. A circular manifold provides an efficient means for oilcompensation.

In an embodiment of the invention at least one one-way valve is arrangedwithin the circular manifold, restricting the circulation in saidmanifold to one direction. In this embodiment the order of stopping thecompressors corresponds to the direction of circulation defined by theat least one one-way valve. This improves the efficiency of oilcirculation as it avoids that a portion of the lubricating oiloscillates between two adjacent compressors.

In an embodiment of the invention the order of stopping the compressorsis reversed every time the predetermined time of the operation of allcompressors is reached. This provides an efficient way of oildistribution among the compressors.

In an embodiment of the invention the period between starting acompressor and stopping the next compressor is larger than thepredetermined period of time for oil compensation. This ensures aconstant output of the set of compressors as it allows the firstcompressor to reach its nominal performance before the next compressoris switched off.

In an embodiment of the invention the fluid connection comprises atleast one trap. Such a trap avoids that gas is flowing into the fluidconnection producing bubbles which may hinder the oil compensation.

In an embodiment of the invention at least on of the compressorscomprises an inspection glass and the fluid connection opens into thecrank case of said compressor at substantially the same height at whichthe inspection glass is positioned. This allows to monitor the level ofoil within the crank case of said compressor.

In an embodiment of the invention the set of piston compressors isconfigured such that, when all compressors are stopped, the height ofthe oil within the crank cases will adjust to a height within thevisible range of the at least one inspection glass. This allows toeasily check the level of oil within the crank case.

A set of piston compressors according to the embodiments of theinvention as described above can be used for cooling refrigerating salesfurnitures. Said refrigerating sales furnitures can be sales furnituresbeing refrigerated to temperatures of above 0° C. or freezing furnituresbeing refrigerated to temperatures of below 0° C.

The features, embodiments and advantages as described with respect tothe set of piston compressors can also be realized in terms of methodsteps, with a method for fabricating the set of piston compressorsaccording to the invention.

While the invention has been described with reference to exemplaryembodiments, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition many modifications may be made to adopt a particular situationor material to the teachings of the invention without departing from theessential scope thereof. Therefore, it is intended that the inventionnot be limited to the particular embodiment disclosed, but that theinvention include all embodiments falling within the scope of thedependent claims.

1. A set of piston compressors for use in a refrigeration system,comprising: a fluid connection between the crank cases of the pistoncompressors for allowing oil compensation between said pistoncompressors; and a control unit which in operation monitors a runningtime of each piston compressor and, in case all piston compressors arerunning, subsequently interrupts the operation of a respective pistoncompressor for a predetermined period of time for oil compensation (TC)if its running time reaches a predetermined maximum running time (TO).2. The set of piston compressors of claim 1, wherein at least one of thepiston compressors is constant speed-piston compressor, and wherein atleast one of the piston compressors is a speed controllable pistoncompressor.
 3. The set of piston compressors of claim 2, furthercomprising at least two speed controllable piston compressorsselectively connectable to a speed control for controlling the speed ofsaid compressors, wherein the control unit in operation sequentiallyinterrupts the operation of each of the speed controllable pistoncompressors not connected to the speed control for a predeterminedperiod of time for oil compensation (TC) after a predetermined delaytime (TS) after the speed controllable piston compressor which has beenstopped before has been started again. 5-19. (canceled)
 20. The set ofpiston compressors of claim 1, wherein the control unit in operationstarts over in counting the running time of a respective pistoncompressor, when said piston compressor is stopped for a period of timethat is longer than the predetermined period of time for oilcompensation (TC).
 21. The set of piston compressors of claim 1, whereinthe control unit in operation stops a first piston compressor, whichreaches the predetermined maximum running time (TO) while a secondpiston compressor has been stopped for oil compensation, after apredetermined delay time (TS) after said second compressor has beenstarted again.
 22. A set of piston compressors for use in arefrigeration system, comprising: a fluid connection between the crankcases of the piston compressors for allowing oil compensation betweensaid piston compressors; and a control unit, which in case all pistoncompressors are running, monitors a common running time of the pistoncompressors, and, when the common running time reaches a predeterminedmaximum running time (TO), wherein the control unit sequentiallyinterrupts the operation of a each piston compressor for a predeterminedperiod of time for oil compensation (TC) while the others continue torun.
 23. The set of piston compressors of claim 22 wherein each pistoncompressor is stopped for oil compensation after a predetermined delaytime (TS) after the compressor which has been stopped before has beenstarted again.
 24. The set of piston compressors of claim 22, whereinthe fluid connection is a circular manifold.
 25. The set of pistoncompressors of claim 24, wherein at least one one-way valve is arrangedwithin the fluid connection, restricting the circulation in said fluid26. The set of piston compressors of claim 23, wherein the order ofstopping the compressors is reversed every time the predetermined timeof the operation (TO) of all compressors is reached.
 27. The set ofpiston compressors of claim 22, wherein the period between re-starting acompressor and stopping the next compressor is larger than thepredetermined period of time for oil compensation (TC).
 28. The set ofpiston compressors of claim 22, wherein the fluid connection comprisesat least one trap.
 29. The set of piston compressors of claim 22,wherein at least one of the piston compressors comprises an inspectionglass and the fluid connection opens into the crank case of the pistoncompressor at the height at which the inspection glass is positioned.30. The set of piston compressors of claim 29, which is configured suchthat, when all compressors are stopped, the height of the oil within thecrank cases will adjust to a height within the visible range of the atleast one inspection glass.
 31. A method for oil compensation in a setof piston compressors, comprising the steps of: monitoring a runningtime of each piston compressor and, in case all piston compressors arerunning, interrupting the operation of a piston compressor for apredetermined period of time for oil compensation (TC) when its runningtime reaches a predetermined time of operation (TO).
 32. A method foroil compensation in a set of piston compressors, comprising the stepsof: monitoring a running time of the piston compressors and, in case allpiston compressors are running and the running time reaches apredetermined time of operation (TO), interrupting the operation of afirst piston compressor for a predetermined period of time for oilcompensation (TC); and sequentially interrupting the operation of eachadditional piston compressor for a predetermined period of time for oilcompensation (TC).
 33. The method for oil compensation of claim 32wherein each piston compressor will be stopped for oil compensationafter a predetermined delay time (TS) after the compressor which hasbeen stopped before has been started again.
 34. The method for oilcompensation of claim 33, wherein the predetermined delay time (TS)between re-starting a first compressor and stopping a second compressoris longer than the predetermined period of time for oil compensation(TC).